HashMap vs. Hashtable in Java: When Should You Choose Which?

HashMap vs Hashtable: Key Differences in Java

Introduction

In Java, both HashMap and Hashtable are versatile data structures used to store key-value pairs. However, there are some key differences between them that can impact their suitability for specific applications.

Synchronization

The primary distinction lies in their synchronization behavior. Hashtable is a synchronized data structure, meaning that all of its operations are thread-safe. This comes at the expense of performance, as thread synchronization can slow down operations in non-threaded applications.

HashMap, on the other hand, is an unsynchronized data structure. This makes it inherently faster for non-threaded applications, which do not require the additional overhead of thread safety.

Null Values and Keys

Another difference is in their handling of null values and keys. Hashtable does not allow null keys or values. In contrast, HashMap allows one null key and any number of null values. This flexibility can be advantageous in certain scenarios.

Iteration Order and Subclasses

HashMap and Hashtable also differ in terms of iteration order. HashMap does not maintain any particular insertion order, while Hashtable maintains the order of key-value pairs as they were inserted.

Additionally, HashMap has a subclass called LinkedHashMap, which provides predictable iteration order. If predictable iteration order is a requirement, you can easily swap out HashMap for LinkedHashMap. This option is not available with Hashtable.

Recommendation

For non-threaded applications where synchronization is not a concern, HashMap is typically the more efficient choice due to its unsynchronized nature. However, if synchronization becomes a necessity, you may consider using ConcurrentHashMap, which provides thread safety without the performance overhead of Hashtable.

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